Superconductive compositions having the formula EQU MBa.sub.2 Cu.sub.3 O.sub.x
where M is selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, and x is from about 6.5 to 7.0 have been subjected to intensive investigation. It is expected that such compositions will find extensive use in a number of practical applications. For use in magnets and power transmission lines, it will be important that such materials be in the form of fibers (also referred to as wires) of high critical current densities (J.sub.c).